Such absorbers have already been known (DE 36 15 360 C2, WO 96/08812, DE-OS 27 24 172, DE 33 13 001 A1). Therein, honeycomb-like chambers in the basic constructional unit, which consists of a supporting body and spacers and is produced via injection-moulding or an RIM-process, are covered in the direction of the sound source by a covering layer. The covering layer comprises burl-like air chambers so that there occurs frictional damping when the "air cushions" vibrate and hit the edges of the spacers or it is smoothly tentered over a porous basic unit consisting of foamed plastic, in particular, and is welded therewith at certain locations.
It is the object underlying the invention to simplify an absorber of this type as regards its production and to improve it as regards its function. Furthermore, its disposability shall, if possible, be improved in that reusable material parts are used, in particular.
The invention is characterised in claim 1 as regards the absorber and in claim 8 as regards the production method. Further improvements are claimed in the subclaims.
The absorber is produced from two respectively integral units. The spacers and the supporting body constitute a preformed basic constructional unit which consists more particularly of deep-drawn or transfer-moulded thermoplastic like polypropylene (PP), pressed thermoplastic or thermosetting plastic, injection-moulded thermoplastic or thermosetting plastic, relief-pressed or, respectively, relief-injected plastics, namely with fibre reinforcements, in particular. These materials may be produced both from new and recycled products.
The following deformation processes may be used for producing the basic constructional unit, in particular:
1. Low Pressure Technology (LPT), wherein plastic is pressed into the respective mould at low pressure between about 10 and 100 bar. Long glass fibers (LFG) having a fiber length of about 12 mm may also be used. In heating duct technology, PP (polypropylene) in particular is mixed with 20% of talcum. PA0 2. Reaction Injection Moulding (RIM), wherein PU (polyurethane) is preferably used as the plastic. In the case of fiber reinforcements (R-RIM) glass fibers are used in order to promote thermal stability. Transfer moulding is carried out at a low pressure of up to about 15 bar. PA0 3. Resin Transfer Moulding (RTM), wherein plastic, more particularly epoxy, phenolic, vinyl resins, optionally reinforced with glass fibers, are pressed into the closed mould, preferably at a low pressure of up to about 20 bar. PA0 4. Injection Moulding, wherein high pressure between about 350 and 700 bar is used for filling the polymer plastic into the casting mould. Reinforcements may equally be used. PA0 5. Glass-mat reinforced thermoplastic method (GMT or AZDEL), wherein PP, for instance, is press-moulded with a glassfiber mat at pressures between about 150 and 200 bar.
For relief-pressed plastic, the plastic is press-moulded with a textile structure or a film between press moulds via LPT or GMT whereas, for relief-injected plastic, the plastic is injected onto the textile structure or the film in an injection mould.
On the other hand, the thin covering unit is a particularly thin covering layer which is pressed directly onto the basic constructional unit from a straight plane and deformed in the process, but which is able to vibrate in certain regions at least. The vibrant regions should respectively be situated between those locations of the covering unit which are substantially rigidly connected with open front ends of spacers, via plastic welding connections in particular. Such welding connections may be realised in that the thermoplastic is heated by means of heat rays, for instance, but also via RF or ultrasonic welding. The covering layer closes the individual resonance chambers both outwardly and between each other.
The covering layer for instance consists of a thin layer of thermoplastics like polypropylene, for instance. However, it may also consist of a fleece. It is preferably configured as a composite material like a laminate of different individual layers. The layer thickness should be situated in the range between 0.5 and 8 mm.
It may be useful for some applicational cases to produce the covering layer from a double layer which on the one hand consists of a thin metal layer of aluminium, in particular, having a thickness between 5 and 200 .mu.m and on the other hand of a thermoplastic plastic like polypropylene, for instance, and/or a fleece which is attached to the side of the aluminium layer facing the basic constructional unit. This thermoplastic layer than constitutes the connecting layer towards the edge of the supporting body on the one hand and towards the front ends of the spacers on the other hand.
The two constructional units are preferably produced via the following process:
First a flat blank or a flat web of the single-layered or multi-layered covering layer is tentered in a straight plane over the preformed basic constructional unit. Subsequently, a stamp is pressed onto the covering layer in the direction towards the basic constructional unit. The stamp front face comprises sort of a "countersurface" against the surface which the covering unit is to occupy in the final state of the basic constructional unit. Thus, the stamp serves as a deformation tool since it deforms the covering layer and presses it onto the open front ends of the spacers and the edge of the basic constructional unit. Due to this deformation, the covering layer assumes the moulded shape wherein it constitutes the covering unit for the absorber. Said layer is simultaneously attached to the basic constructional unit so that it does not "spring back" into its flat initial state.
During this deformation process, the thin covering layer is set onto that side of the basic constructional unit which comprises the open front edges of the spacers between which the intermediate cavities are situated.
Attachment of the covering unit, i.e. connection of the adjacent portions, namely of the edge of the supporting body and open front ends of the desired spacers with the supporting body is more particularly effected via the use of pressure and heat in the case thermoplastic materials or thermoadhesives. The covering unit may as well be provided with a self-adhering adhesive layer on that side which faces the basic constructional unit, namely the edge and the open front ends of the spacers.
The process feature reading that the deformation of the covering layer and the rigid connection thereof with the preformed basic constructional unit should be effected in the very same tool immediately subsequently in time, which is above all helpful in reducing the production time and decreasing the loss of heat energy.
The advantage of the rigid connection also resides in that a characteristic, which might possibly be inherent to the covering layer, namely a more or less rapid, renewed deformation from the preformed configuration into the initially flat configuration in the course of time, is counteracted. In this respect, there exist more possibilites to use different materials. However, it is essential that the covering unit may vibrate freely, at least in regions between the front ends or, respectively, the front edges of the spacers, and that sound waves are transferred into the intermediate cavities so that sound energy may be dampened or, respectively, absorbed within the chamber-like intermediate cavities between the basic constructional unit on the one hand and the covering unit on the other hand.
Instead or in addition thereto, the covering unit may also be provided with holes enabling sound waves to pass through.